An ink-jet printer of the type frequently referred to as drop-on-demand, has at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead, the ink is contained in a plurality of channels. Power pulses-cause the droplets of ink to be expelled as required, from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the power pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in that particular channel and ink bulges from the channel orifice. At that stage, the bubble begins to collapse. The ink within the channel retracts and separates from the bulging ink thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium. The channel is then refilled by capillary action, which in turn draws ink from a supply container of liquid ink. Operation of a thermal ink-jet printer is described in, for example, U.S. Pat. No. 4,849,774.
The ink-jet printhead may be incorporated into either a carriage-type printer or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead is usually sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is attached to a carriage which is reciprocated to print one swath of information (equal to the length of a column of nozzles) at a time, on a stationary recording medium, such as paper or a transparency. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath, or a portion .thereof, so that the next printed swath is contiguous or overlapping therewith. The procedure is repeated until the entire page is printed. In contrast, the page-width printer includes a stationary printhead having a length equal to or greater than the width of the paper. The paper is continually moved past the page-width printhead in a direction normal to the printhead length and at a constant speed during the printing process.
Many liquid inks and particularly those used in thermal ink jet printing, include a colorant or dye and a liquid which is typically an aqueous liquid vehicle, such as water. Some thermal ink jet inks also include a low vapor pressure solvent. When a substrate or a sheet of paper is printed with ink-jet ink, the ink is deposited on the substrate to form an image in the form of text and/or graphics. Once deposited, the liquid is removed from the ink and the paper to fix the colorant to the substrate. The amount of liquid to be removed, of course, varies with the amount of ink deposited on the substrate. For instance, the amount of liquid to be removed from a sheet of paper with 10% printing, as in text only printing, is quite small. If the sheet is covered with 90% printing, however, as when a graphic image is printed, the amount of liquid to be removed is substantially more and can cause image deformation and paper deformation if not properly removed.
The liquid component can be removed from the ink and printed substrate by a number of methods. One simple method is natural air drying in which the liquid component of the ink deposited on the substrate is allowed to evaporate and penetrate into the substrate naturally without mechanical assistance. Another method is to send the printed substrate through a dryer to actively evaporate the liquid. Active drying of printed substrates includes infrared heating, conductive heating and heating by microwave energy. In some cases, a special paper is used in which the liquid is absorbed by a thin coating of absorptive material deposited on the surface of the paper. Blotting of the printed substrate is also known.
While active drying is not essential, it is necessary in high speed printing, where printed images must be actively dryed to enable faster printing speeds. The absorption and desorption of water into and out of the paper, however, has some undesirable side effects, such as strike through, leathering at the edges of the printed image, paper cockle, and paper curl. Paper curl is a function of the amount of liquid deposited per unit area of recording medium. Less printing on a page has less potential to develop curl due to the smaller amount of liquid. More printing on a page has more curl potential due to a higher amount of liquid per unit area.
Generally, the term "curl" refers to the distance between the base line of the arc formed by a recording sheet when viewed in crosssection across its width (or shorter dimension--for example, 8.5 inches in an 8.5.times.11 inch sheet, as opposed to length, or longer dimension--for example, 11 inches in an 8.5.times.11 inch sheet) and the midpoint of the arc. This type of curl applies to long grain cut paper, since curl is typically perpendicular to the process direction of a paper making machine. To measure curl, a sheet can be held with the thumb and forefinger in the middle of one of the short edges of the sheet (for example, in the middle of one of the 8.5 inch edges in an 8.5.times.11 inch sheet) and the arc formed by the sheet can be matched against a pre-drawn standard template curve. Such curl measurement is referred to as "hanging radius curl."
Application of liquid inks, containing water, to paper causes an initial hydroexpansion of the fibers of the paper. This initial hydroexpansion causes an expansion curl away from the image which occurs typically right after printing. Steady state curl, also known as cool curl, is toward the image, and typically occurs over a period of time when the sheet tries to achieve a state of final stress release after being dryed. Active drying accelerates the effect of towards the image curl or steady state curl.
The amount of time that it takes for a sheet of paper to reach steady state, depends on the amount and type of penetrants, co-solvents and humectants which are inherent in the ink being used. Penetrants change the depth of ink penetration and thus modify the amount of time taken to reach steady state curl. It is also known that co-solvent humectant ethylene glycol inks which contain no penetrant or salt remain essentially flat. Ethylene glycol inks, however, appear to produce a long term hydroexpansion that reduces or counteracts the stress relaxation of the sheet. This effect, however, is lost over time to produce a final towards the image curl.
In U.S. Pat. No. 4,853,255 to Onishi et al., a process for controlling curl in a web of coated paper is described. A paper web is unwound from a reel and coated with a coating composition applied to one side of the web. Water is applied to the opposite side of the coated paper to thereby control curl resulting from the coating composition previously applied.
U.S. Pat. No. 5,277,965 to Malhotra, describes a recording sheet which includes an ink receiving layer, a base sheet, a heat absorbing layer, and an anticurl layer. The recording sheet exhibits little or no curling, even after exposure to heat and/or a wide range of relative humidities.